1. Field of the Invention
The present invention relates to an image forming method and apparatus, and more particularly, to technology in a two-liquid reaction system which forms an image on a recording medium by using an ink and a treatment liquid, for achieving high image quality by preventing the occurrence of offset in a fixing step for fixing the image, and the occurrence of curling of the recording medium, and the like.
2. Description of the Related Art
An inkjet recording system performs recording by ejecting and depositing droplets of ink onto a recording medium from a plurality of nozzles formed in an inkjet head, and such a system is able to record images of high resolution and high quality, with little noise during the recording operation and low running costs. The ink ejection system may be, for example, a piezoelectric system, which uses the displacement of a piezoelectric element, a thermal system, which uses thermal energy generated by a heating element, or the like.
In the inkjet recording system, when ink droplets are consecutively deposited in such a manner that the ink droplets (ink dots) that are mutually adjacent on the recording medium overlap with each other, these ink droplets combine together due to their surface tension and give rise to a problem of bleeding (landing interference) in which the desired dots cannot be formed.
In the case of dots of the same color, the dots shape is disturbed and in the case of dots of different colors, an additional problem of color mixing occurs. In particular, when recording with a single-pass system using a line head, the difference in the landing time between mutually adjacent ink droplets is short and therefore landing interference is liable to occur and it is difficult to form a sharply defined image.
In response to this, technology is known which achieves high image quality by depositing a so-called treatment liquid onto a recording medium prior to the ink liquid, and causing this treatment liquid to react with the ink. When using pigment particles as the coloring material, the treatment liquid has the function of aggregating the pigment particles by neutralizing the Coulomb repulsion of the particles and thereby increasing the viscosity of the ink liquid. Thereby, interference between deposited dots is suppressed and sharply defined images can be recorded without the occurrence of non-uniformities in density.
Furthermore, it has also been proposed that glossiness and wear resistance (fixing properties) be imparted to the printing surface by pressing a heated roller or fixing belt (fixing member) against the recording medium with a prescribed pressure after forming an image on the recording medium. However, when pressing a roller, a problem known as “offset” occurs in that a portion of the coloring material on the recording medium adheres to the fixing member. This problem of “offset” is liable to occur with aqueous inks which use an aqueous solvent. Japanese Patent Application Publication No. 2003-131506 discloses, as a countermeasure to “offset”, that a separating layer made of a silicone resin is arranged on a fixing belt, and an adhesiveness improving layer is arranged so as to achieve separation between the separating layer and the recording medium (paper).
However, although it is possible to prevent offset to a certain degree by adjusting the surface material of the fixing member, as in Japanese Patent Application Publication No. 2003-131506, it cannot be regarded as sufficient. In particular, when forming images at high speed, the paper must make contact with the fixing member while there is solvent still remaining on the surface of the paper, and in such cases, offset occurs even if the properties of the surface material of the fixing member are adjusted.
Furthermore, if an aqueous ink is used, there is a problem in that curl is liable to occur in the recording medium, and this issue must be resolved at the same time.